In such networks, compressing the signal transmitted on some links is known in itself. This can be the case in particular on links that have only one channel, to enable the routing of a greater number of calls. Compression can have the drawback of causing a loss of quality and, sometimes, of increasing transit time because of the time needed for compression and decompression. A large number of compressions and decompressions can cause echoes and attenuation.
Some prior art routing methods are unaware of the number of compressions and decompressions and accept deterioration of call quality if the number is high. In some instances quality is degraded. Other routing methods use a static routing table, in which the number of compressions and decompressions is limited. This solution has limitations and the private network cannot be exploited to the full using that solution. A final solution is to use the configuration facility to limit the number of transit nodes and therefore the number of compressions and decompressions; that solution is not applicable in all private networks and limits their configurations.
The problem of overflow also occurs in private networks, i.e. the problem of a call request that cannot be satisfied by the network because its resources are congested. This can happen if the private links of the private network are of fixed capacity, rather than of a capacity that is allocated dynamically and which is less than the maximum volume of traffic. Completing the corresponding call by way of the public network or some other external network is known in itself. In other words, if a user at a first node of the private network wishes to call a user at a second node, and if at least one link of the private network is congested so the call cannot be completed, the call is completed directly from the first node to the second node via some external network—typically the public network.
This solution gives rise to the following problems. Firstly, using the public network incurs a cost; secondly, it is not certain that there is a public network access circuit group for all the nodes. Moreover, from the economic point of view, that solution is not very cost effective, and it does not exploit the resources of the private network to the full.